The transmission capacity of the optical communication has been rapidly increasing every year, and the wavelength division multiplex (WDM) has been put into practical use as a low-cost, high-speed and large-capacity transmission technology to meet such a requirement. The WDM is a technology for transmitting different signals at respective wavelengths by simultaneously using a number of monochromatic lights (several ten to hundred wavelengths) whose frequencies differ by only 50 GHz or 100 GHz, and since the transmission capacity per one fiber can be increased several tens of times or more, the fiber installation cost can be significantly reduced.
Conventionally, a wide variety of semiconductor lasers having different wavelengths and a modularized device (hereinafter, abbreviated as module) for driving them have been required for the light source of the WDM. When manufacturing lasers, crystal has to be made for each wavelength and also a module has to be manufactured for each wavelength, which poses a cost problem. For its solution, a wavelength variable module capable of freely varying a wavelength has been developed. Since this wavelength variable module can vary the light wavelength in a range of about 40 nm, what is needed is to manufacture light-emitting elements of several wavelengths and a module, and it is possible to provide a module at low cost, so that it has become a main light source of the WDM.
In the field of the optical measurement, in particular, in the field of the biological optical measurement, with the further diversification of the substances to be observed, a wavelength variable light source with a wide range of about 0.7 to 1.6 μm has been required. As specific applied fields, nonlinear optical imaging, optical tomography for visualizing the cancer and the immune system and the optoacoustic technique can be enumerated.
Various types of wavelength variable lasers have been examined up to now. In the communications use, an example of a wavelength variable laser of an external cavity type using a liquid crystal filter can be cited. FIG. 12 shows the configuration thereof (see JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 8, Aug. 2006, P3202-3209 (Non-patent Document 1)). The wavelength variable laser shown in FIG. 12 is constituted of a liquid crystal filter 101, a glass etalon 102, a lens 103 and a gain chip 104. The gain chip 104 has a structure in which a gain multilayer 106 for generating an optical gain, a phase control multilayer 107, a rear surface electrode 108, a gain current electrode 109, a phase control current electrode 110 are formed on an InP (indium phosphide) substrate 105. The light emitted from the gain chip 104 is converted into parallel light by the lens 103 and is incident on the glass etalon 102 and the liquid crystal filter 101. The light is reflected by the liquid crystal filter 101 and is incident on the gain chip 104 through the same optical path, thereby constituting the laser cavity. Since the liquid crystal filter 101 and the glass etalon 102 have the wavelength selectivity and the reflection wavelength of the liquid crystal filter 101 can be controlled by voltage application, the wavelength variable laser is obtained. This wavelength variable laser can emit all the wavelength range of the so-called C band used in the optical communication.
Also, as the wavelength variable laser for use in the optical measurement, titanium sapphire laser, an optical parametric oscillator and others have been used.